1. Field of the Invention
This invention relates to a mount device of an interchangeable lens mounted on a camera, and particularly to a mount device capable of transmitting the focal length information of the interchangeable lens to the camera.
2. Description of the Prior Art
For a camera, particularly, a single lens reflex camera, there are usually prepared several types to many types of interchangeable lenses having different focal lengths. When photography is to be effected by the use of these interchangeable lenses, it is sometimes necessary to transmit the focal length information of a mounted lens to the camera in order to obtain more appropriate photographs. Especially, in a camera provided with an electric circuit for metering and operating an object light passed through the lens and determing a combination of shutter speed and aperture value for obtaining a proper exposure, on the basis of a preset program, it is desirable to correct the characteristic of the program by taking the focal length of the interchangeable lens into account. For example, considering the relation between the image magnification and hand vibration, it is desirable in an interchangeable lens haing a long focal length to correct said characteristic to a characteristic having attached importance to a higher shutter speed in order to eliminate the influence of hand vibration.
The technique of transmitting the focal length information of an interchangeable lens to a camera is known from Japanese Patent Publication No. 12429/1966. According to this prior art technique, an interchangeable lens is provided with a signal member comprising a projection having an amount of projection corresponding to the focal length of the lens and, when the lens has been mounted on a camera, the focal length information is transmitted to a camera by said projection. Considering commercially available various interchangeable lenses, there are very many types of interchangeable lenses having relatively short focal lengths such as 8, 10, 15, 16, 20,24, 28, 35, 45, 50, 55, 85 and 105 mm, whereas there are a few types of interchangeable lenses having long focal lengths such as 200, 300, 400, 500, 1000 and 2000 mm. Accordingly, if the amount of projection of the signal member is selected simply to correspond to the focal length, for example, an amount proportional to the focal length, then the information of many lenses of short focal lengths will be condensed within the range of about 1/20 of the maximum amount of projection of the signal member allowable in designing and thus, the signal reading error on the camera side will become great.
What has been stated above equally applies to a case where an interchangeable lens is equipped with a conversion lens to increase the focal length thereof by M times.